Radiophysics of Jupiter
Radiophysics of Jupiter
复制标题
木星的放射物理学
DOI：
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发表时间：
1967
期刊：
影响因子：
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通讯作者：
J. Warwick
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作者：
J. Warwick
结论对于 DIM 来说，尽管 Roberts 和 Ekers 以及 Berge 做出了很大的努力，但固定发射质心的问题仍然存在。 DIM 和 DAM 存在不对称性，其唯一的解释是偶极子位移。令人满意的答案可能取决于弧秒笔形光束实时进行的研究。人们可能会辨别出嵌入辐射带发射晕中的木星圆盘的热发射。然后观察结果将进行自我校准。这种测量需要具有 5 弧秒数量级的多笔光束的设备。 Io 和 Amalthea 对 DIM 也可能产生明显的局部影响（更确切地说，未发表）。DIM 更直接的问题当然包括 - 旋转周期的细化，因为它与 DAM 的旋转周期明显不一致。 Barber (1966) 和 Dickel (1967) 认为该周期与系统 III (1957.0) 周期相差 0.2 秒以内。自转周期的定期检查似乎很重要，并且可以使用相对简单的设备进行。现在世界各地的许多台站都在观测 DAM，尽管集中度最高的是美国。以有限的频率进行 24 小时天气覆盖是有价值的（Alexander，1966）。如果这样的研究可行的话，它可能会更早地暗示木卫一的调制。目前 DAM 观察者以摘要形式发布数据的趋势也是值得推荐的（例如，参见 Morrow、Barrow 和 Resch，1965 年的目录）。然而，所需的主要信息是更精细的数据，特别是关于毫秒突发的快速分辨率偏振测量和光谱学。阿雷西博记录的这些爆发的极化多样性需要确认。博尔德正在安装设备以继续研究，但可能会因缺乏天线收集区域而受到影响。此外，我们计划将扫频接收器扩展到更高的频率，从 40 Mc/s 到 80 Mc/s。需要以高灵敏度连续覆盖该范围才能确定可能的局部发射光谱岛的存在。迄今为止进行的任何 DAM 调查都无法发现这些问题。木星爆发的电离层法拉第效应应该以比迄今为止所达到的更高的精度进行观测。此类研究的一个可能结果可能是检测木星自转对 DAM 偏振椭圆方向的影响。木星的雷达观测为未来带来了很多希望。来自该软目标的回波检测显然是可变的（Pettengill，1965）。 Pettengill (1966) 还指出，提高雷达系统功率可能会在未来十年内检测到来自木星伽利略卫星的回波，并建议在卫星被木星电离层遮挡时测量极化。这样的测量可以提供对木星磁场的独立确定。空间观测如果，很可能是这样，DAM是在木星电离层和磁层的电子陀螺仪频率附近生成的，则发射频率的下限由包含发射粒子或波的最弱场给出。这些场位于木星磁层的最外层，其范围不确定（例如，10-50个木星半径；在磁层尾部，距离更大；这个结构无疑对我们的天空有一定的度数！）。在磁层顶，陀螺仪频率约为 100 cps，在 Io 时，陀螺仪频率为 150 kc/s。 1个电子cm-3对应的行星际等离子体频率为9kc/s。如果将敏感的射电望远镜放置在地球磁层之外，对木星射电发射下限的观测可能会成功。木星辐射带的观测有关木星非热现象的所有已知事实都来自射电天文数据。对于大多数射电天文学家来说，对推断的粒子和场进行原位验证似乎是空间研究的优先事项。设计并将装置飞向木星并不容易。这些设备不仅必须能够承受数年的长途航行，而且还必须能够承受到达时明显恶劣的环境。这样的飞行可能带来的好处是：（1）更深入地了解高能粒子产生和加速的等离子体物理过程；（2）了解等离子体无线电发射的非线性机制；（3）了解旋转体中磁场的起源；（4）在太阳系内完全不同的地方观测太阳风和高能粒子。
诸如这些在内行星系统内合理的太空探测器等目标。木星探测器，尤其是木星轨道飞行器，增强了回报的前景，因为除了地球之外，这颗行星已知涉及感兴趣的现象。木星磁场的不对称性有关木星磁场对称性的数据仍然存在明显的不一致。每个人都同意 DAM 要求行星场偏离中心偶极子。我发现有必要回顾一下我的怀疑理由，尽管罗伯茨和埃克斯进行了出色的测量，但这些偏离的性质目前尚未得到解决。如果 DIM 的质心位于质量中心，则 DIM 极化方向作为经度函数的明显不对称性需要解释，强度变化作为地球平磁纬度函数也需要解释。目前，除了辐射带南移的行星阴影之外，还没有对偏振效应提出任何解释。现在看来（第 3.2 节），强度效应表明木星的磁场是一个非常纯净的偶极子。因此，偶极子场的畸变会产生极化效应的建议无法得到支持。在第 3.2 节中，我们表明 Roberts 和 Komesaroff (1965) 将强度确定为纬度的函数，在平磁纬度 +1.2° 周围是对称的。就相对论电子的镜像而言，该纬度代表木星的有效磁赤道。假设观察到的不对称性源自由轴对称四极场添加到预先存在的偶极场组成的磁场。镜点赤道（存在最小磁场的地方）位于偶极子赤道以北。为了实现这一点，木星磁轴的南极必须具有更强的磁场（与 DAM 的位移偶极子模型一致）。如果磁场是由轴对称四极场加上偶极场组成，则四极具有负极（具有向内指向的磁力线）和正中心（具有向外指向的磁力线）。详细计算表明，对于距离木星中心 2.5 个半径的同步加速器发射，如果四极与偶极矩之比以木星半径为单位为 0.018，则可以实现所需的不对称性。如果偶极子和四极子都位于木星中心，则赤道场强之比为 0.0552。南方极地场比北方稍强，但差异太小，无法解释 DAM 的不对称性或 DIM 的极化不对称性。地球的四极场是偶极场的0.08，并且地球也有相当强的更高极分量。换句话说，木星场似乎是一个比地球场更纯粹的偶极场。在四极模型中，北半球和南半球的地磁纬度之间存在差异，其中穿过艾奥的力线与木星表面相交。然而，这种差异仅约为四度，再次强调了木星偶极子场的纯度。
ConclusionsTypes of data that are neededFor DIM, the problem of fixing the emission centroid remains, despite the very strong efforts by Roberts and Ekers, and by Berge. There are asymmetries in DIM and DAM, whose only explanation has been in terms of the displaced dipole. A satisfactory answer may depend on studies carried out in real time by a second-of-arc pencil beam. One might discern the thermal emission from Jupiter's disk embedded in the halo of radiation belt emission. The observations then would be self-calibrating. Such a measurement would require apparatus with multiple-pencil beams of the order of 5 seconds of arc. There might also be discernible local effects of Io and Amalthea on DIM (Rather, unpublished).More immediate problems for DIM certainly include-refinement of the rotational period, in view of its apparent disagreement with DAM's rotation period. Barber (1966) and Dickel (1967) believe that the period lies within 0.2 seconds of the system III (1957.0) period. Periodic checks of the rotational period seem important, and can be carried out with relatively simple equipment.Many stations around the world now observe DAM, although the concentration is heaviest in the U.S.A. There is value in 24-hour synoptic coverage at limited frequencies (Alexander, 1966). Such a study might possibly have suggested Io's modulation earlier, had it been available. The current tendency for observers of DAM to publish their data in summary form is also much to be recommended (see, for example, the catalogue of Morrow, Barrow, and Resch, 1965).However, the principal information needed is more refined data, especially on the fast-time resolution polarimetry and spectroscopy of millisecond bursts. The polarization diversity on these bursts as recorded at Arecibo needs confirmation. At Boulder equipment is being set up for continuing the study, but it may suffer from lack of antenna collecting area. In addition we plan to extend the swept-frequency receiver towards higher frequencies, from 40 to 80 Mc/s. Continuous coverage of that range, with high sensitivity, is required to establish the existence of possible localized spectral islands of emission. These would have escaped detection in any DAM surveys made to date. The ionospheric Faraday effect on Jupiter bursts should be observed with higher precision than so far accomplished. One possible result of such a study might be the detection of the effects of Jupiter's rotation in the orientation of DAM's polarization ellipse.Radar observations of Jupiter promise much for the future. The detection of echoes from this soft target is apparently variable (Pettengill, 1965). Pettengill (1966) also notes that improving radar system power may permit detection of echoes from Jupiter's Galilean satellites in the next decade, and suggests that the polarization should be measured as the satellite is occulted by Jupiter's ionosphere. Such measurements could provide an independent determination of Jupiter's magnetic field.Space observationsIf, as is likely the case, DAM is generated near the electron gyro frequency of Jupiter's ionosphere and magnetosphere, a lower limit of the emitted frequency is given by the weakest field containing emitting particles or waves. These fields lie at the outermost parts of the magnetosphere of Jupiter, whose extent is uncertain (say, 10–50 Jupiter radii; in the magnetospheric tail, the distance is still greater; this structure undoubtedly subtends degrees in our sky!). At the magnetopause, the gyro frequency is about 100 cps, and at Io, 150 kc/s. The interplanetary plasma frequency corresponding to one electron cm-3 is 9 kc/s. Observations of the lower limit of radio emission from Jupiter may succeed if sensitive radio telescopes are placed outside of the earth's magnetosphere.Observations of jupiter's radiation beltsAll known facts concerning non-thermal phenomena at Jupiter derive from radio astronomical data. In situ verification of the inferred particles and fields seems to most radioastronomers to be a priority item for space research. To design and fly apparatus to Jupiter is not easy. The equipment must survive not only a long voyage, several years in length, but also an obviously hostile environment upon its arrival. Benefits that might accrue to such a flight are:(1)deeper understanding of plasma physical processes of generation and acceleration of energetic particles;(2)understanding of non-linear mechanisms for creation of radio emission from plasmas;(3)understanding of the origin of magnetic fields in rotating bodies;(4)observations of the solar wind and energetic particles at radically different places within the solar system.
Objectives such as these justified space probes within the inner planetary system. A Jupiter probe, and especially a Jupiter orbiter, enhances the prospects of a pay-off, because this planet, uniquely aside from the earth, is known to involve the phenomena of interest.Asymmetries in Jupiter's magnetic fieldThere remains an outstanding inconsistency in the data on the symmetry of Jupiter's magnetic field. Everyone agrees that DAM requires departure of the planetary field from a centered dipole. I find it necessary to review my reasons for suspecting that the nature of these departures is not resolved at present, despite the wonderful measurements by Roberts and Ekers. If the centroid of DIM is at the mass center, the evident asymmetry of the direction of DIM's polarization as function of longitude requires explanation, as does the variation of intensity, as a function of zenomagnetic latitude of the earth. At present, no explanation other than planetary shadowing of southward-shifted radiation belts has been advanced for the polarization effect. It now appears (Section 3.2) that the intensity effect shows that Jupiter's magnetic field is a very pure dipole. The suggestion that distortion of the dipole field produces the polarization effect therefore cannot be supported.In Section 3.2 we showed that Roberts and Komesaroff's (1965) determination of intensity as a function of latitude is symmetric around zenomagnetic latitude +1.2°. This latitude represents the effective magnetic equator of Jupiter, so far as the mirroring of the relativistic electrons is concerned. Assume that the observed asymmetry derives from a magnetic field made up of an axi-symmetric quadrupole field added to the pre-existing dipole field. The mirror-point equator (where the minimum magnetic field exists) lies north of the dipole equator. To achieve this, the southern pole of Jupiter's magnetic axis must have a stronger field (in agreement with the displaced dipole model for DAM). If the magnetic field is made up of an axi-symmetric quadrupole field added to a dipole field, the quadrupole has negative poles (with inwardly directed field lines) and a positive center (with outwardly directed fieldlines).Detailed calculations show that for synchrotron emission at 2.5 radii from the center of Jupiter, the required asymmetry is achieved if the ratio of quadrupole to dipole moment is .018 in units of Jupiter's radius. If both the dipole and the quadrupole lie at Jupiter's center, the ratio of equatorial field strengths is .0552. The southern polar field is slightly stronger than the northern, but the difference is too small to account for the asymmetries of DAM or the polarization asymmetry of DIM. The quadrupole field of the earth is .08 that of the dipole, and the earth has rather strong higher pole components as well. In other words, it seems that Jupiter's field is a more purely dipole field than is the earth's field.In the quadrupole model, there is a difference between the zenomagnetic latitudes in the northern and southern hemispheres where the line of force through Io intersects the surface of Jupiter. However, this difference amounts to only about four degrees, again emphasizing the purity of Jupiter's dipole field.